This invention relates to superconductor composite wire, and more particularly to a process for producing monocore Nb.sub.3 Sn superconductor wire.
A type of superconductor wire that is commonly used comprises a plurality of Nb.sub.3 Sn rods which are imbedded into a matrix material such as copper. There are three known methods for production of such wire. The first is known as the Bronze process, the second is known as the External Sn Over Cu Process or External Bronze Process, and the third is the Internal Sn or Tin Core Process.
The Internal Sn Process can best be demonstrated and explained from U.S. Pat. No. 3,905,839. In this patent, a Nb.sub.3 Sn type superconductor composite wire is assembled by combining Nb wires in a Cu matrix, and utilizing a Sn or Sn-Cu rod core. The Sn is then permitted to diffuse into the Cu matrix and then into the Nb wires through the application of heat. Various examples are presented in this patent for forming the initial assembly after which a heat treatment occurs to diffuse the Sn into the material including the Nb.
In one arrangement described in this patent, a plurality of Nb rods coated with Cu were coaxially arranged around a Cu-Sn alloy center rod. This intermediate composite was then drawn on a draw bench to form a composite wire. The composite wire was then heat treated for several hours to provide diffusion of the Sn into the Cu and Nb to form the Nb.sub.3 Sn wire.
In another example described in this patent, a plurality of apertures formed in a Cu shell were filled with the Nb rods, and a Sn rod was inserted in the center to form the intermediate composite body. The intermediate composite was then heat treated to cause the diffusion, following which the wire was drawn. Other examples were also presented.
However, in all of the process steps described, the composite was first formed after which diffusion occurred through a heat treatment step. It has been found, however, that this process leads to high processing costs and a disturbing incidence of product failure. In addition, the critical current densities available through this product are frequently lower than desired. Furthermore, the wire from this process results in being too brittle to be utilized effectively in coils and other superconductor applications.
A variation on the internal Sn process has been developed by Intermagnetics General Corp. and described in an article entitled "An Overview of the IGC Internal Tin Nb.sub.3 Sn Conductor", published in IEEE Transactions on Magnetics, Vol. Mag 21, No. 1, March 85. The process described can be referred to as a two-step process, and effectively utilizes two sequences of processing procedures. In the first sequence, a group of Nb rods are placed in a matrix of hexagonal Cu cells grouped together in an overall Cu shell. Some 300-1000 Nb wires can be compacted in a shell of this nature. The shell is typically evacuated, welded compacted, and hot extruded. At this point a Sn rod is inserted into the center and the entire composite assembly is wire drawn to form a hexagonal subelement. The second sequence comprises taking a plurality of these subelements and inserting them within a stabilizer tube whose interior surface has been coated with a barrier, such as Nb. The bundle of hexagonal subelements are combined into a hexagonal pattern by utilizing anywhere from 19 to 61 such subelements. The stabilizer tube with the subelement bundle is then again wire drawn until the final wire is formed.
While such two step procedure produces wires of a great numbers of filaments, the cost of conducting the two step procedure adds to the cost of the wire. Additionally, since subelements are first formed and then individually handled to combine them into the final assembly, there is a possibility of causing damage to the subelements as they are combined into the bundle and drawn again.
Even in forming single core conductors, the two-step process had been used, similar to making of a multicore conductor. Such single core conductors made through a two-step process were described in papers presented at the ICMC conference in Cambridge, Mass. in 1985, namely, "Internal Tin Process Nb.sub.3 Sn Superconductors for 18 Tesla" and "The Relationship Between Critical Current and Microstructure of Internal Tin Wire". In both of these the wires used were formed by first forming as separate parts an extruded composite tube of Nb filaments in a Cu matrix and then inserting the Sn core. Then, a single such tube was inserted into a separate stabilizer tube with a barrier layer and then wire drawn.
Accordingly, there is need for an efficient process for producing Nb.sub.3 Sn type superconductor wire which results in high current density, high ductility, reduced costs, simplification of metalurgical procedures, and low damage and failure rates.